In hybrid vehicles and electric vehicles, lithium-ion technology battery packs are used, consisting of a large number of electrochemical battery cells connected in series. Such battery packs, which can comprise a plurality of battery modules, are monitored by means of a battery management system. On the one hand the battery management system monitors the battery pack, which can comprise a plurality of battery modules, and on the other hand guarantees a very long lifetime of the battery modules of the battery pack.
In order to ensure a long lifetime of individual battery modules of a battery pack, the states of charge (State of Charge—SoC) of individual battery cells should be matched to each other despite different self-discharges. This occurs by suitable cell symmetrizing, which is generally carried out resistively, i.e. using at least one resistance, which is also referred to as “cell balancing”. For this purpose, at least one resistance and one switch element are associated with each battery cell in order to be able to specifically discharge individual battery cells by means of said at least one resistance, which is also referred to as a balancing resistance.
Besides different self-discharging rates of the individual battery cells, the capacitances of the battery cells also deviate from each other as a result of production scatter. Said effect is negligibly small at the start of the lifetime, but can increase over the lifetime of the battery cells through differences in the cell ageing and can cause differences in capacitance between the battery cells of up to several per cent to occur.
It is known to use a battery management system for monitoring the states of charge of a battery. It should guarantee, besides safety monitoring, a very long lifetime of the battery and ensure that the states of charge of the individual battery cells are matched to each other. This occurs by suitable cell symmetrization, the so-called “cell balancing”. The cell symmetrization or balancing of the states of charge is generally carried out resistively. For this purpose, a resistance and a switch element are associated with each cell in order to be able to specifically discharge individual battery cells. A device for charge equalization of an energy source with a plurality of cells is known from DE 10 2006 022 394 A1, in which the cells are connected to a discharge unit for charge equalization, which at least partially discharges the battery cells. According to the prior art, however, it is also possible to perform the cell balancing capacitively—i.e. with switched capacitors—or inductively—by means of switched inductance. In these two cases energy can be exchanged between the cells with limited efficiency, whereas with resistive cell balancing the energy can only be converted into heat and is thus lost.
It is known that the maximum allowed charging power decreases with increasing state of charge, whereas the maximum allowed discharging power increases. For these reasons it would be desirable according to the prior art to operate a battery pack for hybrid vehicles or for electric vehicles at a state of charge of 50%. In general in practice, however, an operating window is used, for example between 40% and 60% state of charge. For “plug-in hybrids” the operating window is correspondingly greater, for example 10% to 90% state of charge.
An established balancing strategy seeks to achieve a constantly equal state of charge (SoC) of all battery cells. In order to achieve this, all battery cells are generally symmetrized to identical open circuit voltages. Said strategy is justified for new condition battery cells with almost identical capacitance. For battery cells of different capacitance, however, such as occurs through production scatter and ageing, said balancing strategy leads to unnecessary energy losses through the balancing.
In battery systems in which the capacitance of the individual battery cells is not known and resistive cell balancing is carried out until a common state of charge (SoC) is reached, the total charge to be equalized is very high because charge is unnecessarily discharged via the balancing resistances, which is far greater than the pure equalizing of the different self-discharges of the individual battery cells.
According to the illustration in FIG. 1, two different battery cells 16, 18 of a battery module not shown in detail initially have a state of charge 10 (SoC) of 50%, wherein the capacitance of the first battery cell 16 is less than that of the second battery cell 18. Starting from said state considered in the first step, a charging process 12 of the two battery cells 16 and 18 takes place in the second step, during which the present state of charge (SoC) of the two battery cells 16, 18 rises, as indicated in step 2. During the charging process the current state of charge 24 of the first battery cell 16 rises above the current state of charge 24 of the second battery cell 18. Therefore in step 2 according to the illustration in FIG. 1, discharging 22 of the first battery cell 16 takes place, so that the two states of charge 24 of battery cells that are to be equalized with each other in step 3 are again identical.
Step 4 shows that starting from the state of charge 10 of the two battery cells 14, 16, which corresponds to 50%, a discharging process 14 takes place while the current state of charge 24 of the first battery cell 16 exceeds the current state of charge 24 of the second battery cell 18 by an excess charge, so that as shown in step 4 of the illustration according to FIG. 1, discharging 26 of the second battery cell 18 takes place, so that the two states of charge (SoCs) of the first battery cell 16 and of the second battery cell 18 again equalize with each other and in step 5 are again identical. It can thus be seen from the illustration according to FIG. 1 that for such a continuous charge equalization, i.e. the aim of the objective, that all battery cells 16, 18 have the same state of charge (SoC), not only is unnecessary charge discharged via the balancing resistances required for resistive balancing, but that at the same time an unnecessary number, i.e. an avoidable number, of switching processes of the balancing unit (BCU) occurs. This in turn leads to a significant reduction in the lifetime of the BCUs used for resistive balancing.